Firing pin-initiated primers are employed in artillery ammunition primarily for initiation of the powder charge in initiation fuses, but are also included in different types of impact fuses for shells.
In connection with the increase of, primarily, the rate of fire of anti-aircraft artillery and the introduction of automatic fire to guns of increasingly larger caliber, the need for primers which withstand extreme loading forces has also greatly increased. The term loading forces is used here for those forces which influence a charge or a cartridge when such is loaded in place in the barrel of the gun. Hence, such loading forces consist of a rapid acceleration when the cartridge or charge is thrown into the chamber, and a subsequent equally rapid retardation when it is arrested in its place in the chamber of the barrel.
The requirement that primers be capable of withstanding extreme loading forces is both one of safety, since initiation must not take place on loading, and one of function, since an initiation function which is impaired on loading will in turn influence the V.sub.o of the discharged projectile, that is its muzzle velocity.
A firing pin-initiated primer or percussion cap of the basic type under consideration here consists of a pressed or cast impact-sensitive pyrotechnical charge of a known type, a so-called anvil which abuts against the sides of the primer charge which face in the initiation direction thereof, that is towards the main or propellant charge which is to be initiated by the primer, and a protective case or capsule surrounding the other sides of the primer charge and consisting of at least partly deformable material.
The surface of the primer charge facing the anvil may also be covered by a readily destructible protective foil which, as a rule, mainly has a moisture-protective function. On the initiation of the primer, the case is, thus, to be deformed by a firing pin opposite the anvil so that the primer charge which, in such instance, is compressed between the anvil and the deformed case, is initiated. In the primer designs most commonly employed today, the anvil consists of a bent sheet bridge with gaps on either side thereof in order that the flame jets from the initiated primer charge will be able to reach the main or propellant charge. In other prior art primers, the anvil consists of a metal body perforated by some means for the passage of the flame jets. The drawback inherent in both of these basic types of anvil is that they leave greater or smaller parts of the upper surface of the primer charge wholly without support, either in the form of gaps beside the anvil or perforations through the anvil. Under extreme loading forces, there is, in these types of primer, a risk that the unsupported portion of the primer charge will be pulverized and, in the worst case scenario, this may result in an accidental initiation, but in any event always an uneven initiation with a varying V.sub.o (muzzle velocity) as a consequence.